The themes aren’t that complex. I liked it because it was such a straightforward pageturner. It’s one of the few novels I’ve read in one sitting. I assume the problem with getting Ender onscreen is not its complexity but the fact that it’s incredibly violent and about a 5 year old (or so, I don’t recall perfectly). Also a lot of it takes place in zero gravity.
The book starts with Ender being around 5 and ends with him being 14 or so, so there is some difficulty there, particularly since Ender is a genius and approaches adult-level maturity as a child. It would have been easier as an animated movie.
It is violent in places, with Ender (at around 8) beating to death an older bully. Actually, he does it twice, with the first time being in kindergarten.